Lecture date: Monday, 2014.09.08 (lecture recording)
By 'vacuum processing', we mean processing that occurs at very low pressure.
Atmospheric pressure is 760 Torr; processing occurs at a few millitorr, thousands of times less.
1/4-1/3 of the processes in semiconductor manufacturing are performed at low pressure.
The Overall Process
How is an SiO₂ film created? Grown by exposing the wafer to oxygen!
<image of wafers>
A photoresist polymer film can then be spread across the surface using spin-coating.
A 'mask' (quartz plate, 1/4" thick, with chrome pattern) contains the pattern that we want to reproduce on the wafer.
<diagrams of photolithography>
The mask pattern is thereby transferred to the photoresist, and then transferred to the wafer by etching.
Etching can be dry or wet. Wet etching uses liquid chemicals, but it creates 'undercut' due to its isotropic (equal in all directions) nature:
<image of etching undercut>
Dry (plasma) etching fixes this problem by bombarding the surface with reactive species at a near-perfect perpendicular: so-called anisotropic (unequal in some directions) etching.
<image of dry etched layer>
The photoresist can then be removed (aka 'ashed') by an oxygen plasma, followed by liquid cleaning.
The mask pattern has now been transferred to the SiO2, and the process can continue: ion implanation, thermal annealing, etc.
Where is Vacuum Used?
Metal deposition (sputtering)
Chemical vapor deposition
Plasma photoresist ashing
Basic Vacuum System
Semiconductor reactors are flow systems.
Can you pattern deposition? This was explored with selective nucleation for tungsten deposition... but it never worked that well. Easier to blanket the entire wafer, pattern with photoresist, expose, etch, clean.